In the optical digital audio disc which is a rotatable disc provided with a spiral recording track on which an audio signal digitalized in accordance with the pulse code modulation (PCM) is recorded with constant recording density so as to be optically read out, the digital data signal recorded thereon is formed into a predetermined format which, for example, contains a series of frame units, each of which is formed with, for example, 588 channel bits to include a frame synchronous signal at the beginning and a data portion following the frame synchronous signal. When the digital data signal thus formed is recorded on the optical digital audio disc, the so-called run length limited code modulation is adopted. The run length limited code modulation in a modulation system in which two different levels of the recording signal are provided in accordance with each binary data "0" and each "1", respectively, and the minimum run length (the minimum interval between two successive level transitions) is made relatively long, so that recording efficiency is increased. Moreover, the maximum run length (the maximum interval between two successive level transitions) is made relatively short so that self-clocking in the reproducing process is facilitated. The maximum run length is arranged to correspond to, for example, 11 channel bits and the minimum run length is arranged to correspond to, for example, 3 channel bits. In addition, since the maximum run length does not appear successively in the normal modulated output, a specific pattern of waveform formed with two successive intervals at the levels of "1" and "0" each corresponding to the maximum run length is added to the modulated output to be used as the frame synchronous signal.
In data, signal reproducing apparatus that uses a reading light beam to reproduce a data signal from an optical recording medium, such as an optical digital audio disc, on which a digital data signal having a waveform obtained by the run length limited code modulation, wherein the maximum run length and the minimum run length are fixed appropriately is recorded with constant recording density together with a synchronous signal having the pattern of waveform formed with two successive intervals each corresponding to the maximum run length, the optical recording medium is driven to move so that the recording track of the optical recording medium is scanned at a constant velocity by a reading device, that is, an optical head of the apparatus for emitting the reading light beam, and thereby the digital data signal recorded on the optical recording medium is read by the reading light beam to produce a head output signal.
The head output signal thus obtained is supplied to a level comparing circuit forming a waveform shaping portion wherein the level of the head output signal is compared with a predetermined level (a cut-off level) and shaped into a rectangular waveform taking, for example, a level of "1" when the level of the head output signal is equal to or higher than the cut-off level and a level of "0" when the level of the head output signal is lower than the cut-off level.
In the case of the optical digital audio disc, for example, since the digital data signal recorded thereon is modulated as described above, a reproduced signal with a rectangular waveform obtained by shaping the head output signal from the disc is formed to contain a series of frame periods, each of which commences with a frame synchronous signal formed with two successive intervals each corresponding to the maximum run length Tmax of 11 channel bits and a data portion following frame synchronous signal which includes intervals corresponding to the levels of "1" and "0", respectively, and appearing alternately in response to the contents of the digital data signal recorded on the disc. With this rectangular reproduced signal, the reproduction of bit clock pulses and the detection of the frame synchronous signal are carried out, then the data portion of the rectangular reproduced signal is demodulated with the reproduced bit clock pulses in synchronism with the detected frame synchronous signal to produce demodulated digital data and the demodulated digital data are subjected to error correction and other necessary processing.
In processing such a rectangular reproduced signal as mentioned above, the synchronization with the detected frame synchronous signal is very important. If this synchronization is improper or insufficient, correctly demodulated digital data can not be obtained and proper error correction for the demodulated digital data can not be performed so that noise components arise conspicuously. Accordingly, it is required to detect accurately the frame synchronous signal in the rectangular reproduced signal for processing the rectangular reproduced signal. The detection of the frame synchronous signal in the rectangular reproduced signal is carried out by detecting a specific pattern in the waveform of the rectangular reproduced signal which is formed with two successive intervals each corresponding the maximum run length. It is not difficult to detect accurately the frame synchronous signal in the rectangular reproduced signal when the digital data signal is properly read from the optical record medium by the reading light beam.
However, in the case where the optical recording medium has a light reflective recording surface on which the digital data signal is recorded in the form of variations in geometrical shape, for example, small pits arranged in a spiral track, such as found in an optical digital audio disc, and the digital data signal is read by detecting the variations in intensity of the reading light beam reflected at the light reflective recording surface of the optical recording medium after impinging thereon, if the light reflective recording surface of the optical recording medium has a non-reflecting defective portion, the digital data signal is not properly read by the reading light beam at such a non-reflecting defective portion, as well as at the peripheral portions thereof. This results in a possibility of erroneous detection of the frame synchronous signal caused when the reading light beam scans the non-reflecting defective portion and the peripheral portion thereof, especially, when the reading light beam enters or exists the non-reflecting defective portion.
For example, when the digital data signal recorded on the optical record medium is read properly at the portion of the light reflective recording surface where the pits P are arranged as shown in FIG. 1A, a head output signal Q having the level which varies higher and lower than a cut-off level L in accordance with the existence and absence of the pit P as shown in FIG. 1B is obtained. The head output signal Q is compared with the cut-off level L and waveform-shaped to produce a rectangular signal S having a level which varies in response to the arrangement of the pits P on the light reflective recording surface, as shown in FIG. 1C.
On the other hand, when the portion of the light reflective recording surface where the pits P are arranged as shown in FIG. 1A is located at the front or the rear of a non-reflecting defective portion provided undesirably on the light reflective recording surface, the reading light beam is adversely affected by the non-reflecting defective portion to deteriorate the focusing condition thereof at the portion where the pits P are arranged as shown in FIG. 1A, so that the detected variations in intensity of the reading light beam reflected at the light reflective recording surface do not follow faithfully the arrangement of the pits. Accordingly, the digital data signal recorded on the optical recording medium is not properly read by the reading light beam and a head output signal Q' which is deteriorated in frequency characteristics as shown in FIG. 1D is obtained. The variations in the level of this head output signal Q' to be higher and lower than the cut-off level L do not fully respond to the existence and absence of the pit P, and consequently a rectangular reproduced signal S' which has enlarged intervals T.sub.1 ' and T.sub.2 ' between respective level transitions thereof without varying in level in response to the arrangement of the pits P as shown in FIG. 1E is obtained.
In such a case, there is a high probability that the enlarged intervals T.sub.1 ' and T.sub.2 ' as mentioned above will occur and form a pattern of waveform similar to the pattern of waveform formed with successive intervals each corresponding to the maximum run length, that is, the pattern of the frame synchronous signal in the rectangular reproduced signal S appropriately obtained as shown in FIG. 1C. If that occurs, the enlarged intervals T.sub.1 ' and T.sub.2 ' in the rectangular reproduced signal S' will be erroneously detected as a frame synchronous signal.
In this connection, when the data portion of the rectangular reproduced signal S' is demodulated in the condition of synchronism with the false frame synchronous signal erroneously detected as mentioned above to produce demodulated digital data and then the demodulated digital data are subjected to error correction and other necessary processing, the problem that erroneous output signals, that is, noise components arise conspicuously is caused.